The ratio of light emitted versus the amount of power consumed (also known as efficacy) for early light emitting diodes (LEDs) was relatively poor. Recent advances in LED technology have dramatically increased LED efficacy. For example, some present-day LEDs exceed 100 lumens per watt. In contrast, a conventional incandescent light bulb only produces roughly 17 lumens per watt. In addition to improved efficacy, LEDs also offer greater durability, improved light focusing, and longer life span than incandescent bulbs. Clearly, LEDs are becoming an extremely viable lighting alternative.
One drawback to using LEDs is that, in contrast to incandescent bulbs, which radiate most of their waste heat in the infrared, LEDs do not radiate outside of their emission spectrum. Instead, waste heat must be conducted away through thermal transmission. In other words, LEDs generally require heat sinks to carry the heat away. Excess heat that is not handled properly can cause a shift in the spectral emission of an LED and also lead to premature failure of the LED. For example, some LEDs when detached from their heat sinks will incinerate themselves within a few seconds. Thus, heat management for LEDs is critical. In some cases, simply adding a heat sink to an LED is not sufficient. For example, it is possible that a heat sink may become detached from an LED during operation, causing the LED to overheat and eventually burn out.
Conventional LED lighting applications typically use a driver integrated circuit to power an externally coupled LED. One such circuit is the LM3402/LM3402HV, “0.5A Constant Current Buck Regulator for Driving High Power LEDs,” manufactured by National Semiconductor Corporation. Such conventional driver circuits do not monitor the temperature of an attached LED. Instead, additional external circuitry is required to measure the temperature of the LED. This external circuit may involve, for example, attaching a temperature sensitive element (e.g., thermister, thermocouple, etc.) to the LED itself or, more likely, the heat sink. Because the temperature sensing circuitry is external to the driver IC, it has limited control over the amount of current through the LED. For example, while such circuitry may be able to cut off power to the driver circuit altogether, it is not able to incrementally reduce the current through the LED. This lack of control is unacceptable, for example, in emergency situations where a diminished level of output is desired over no output at all.
In addition to simply overheating, LEDs are susceptible to current runaway. This is due to the fact that as an LED increases in temperature, electrons are allowed to move more freely through it. This results in increased current through the LED, which in turn generates even more heat, and so on. Some conventional circuits monitor the current through an LED and, through feedback, operate to prevent current runaway. For example, in one conventional implementation, a small sense resistor is externally coupled in series with the LED. The voltage across the resistor is measured and thereby used to indirectly determine the current through the LED. While such circuitry may prevent current runaway by cutting back the current, it cannot specifically detect a short-circuit of the LED. Moreover, this circuitry cannot intelligently determine why a reduction in current is necessary. For example, the circuitry cannot detect that a heat sink has become detached, causing an increase in temperature and current of the LED.
Thus, conventional technology does not provide an effective solution for monitoring the temperature of an LED and controlling the current though the LED based on the temperature. Additionally, conventional technology does not allow for detection of a short-circuit or open-circuit through an LED or one or more strings of LEDs.